The present invention relates generally to vertical cavity surface emitting lasers (VCSELs), and in particular to a monolithically integrated VCSEL whose resonator cavity is designed for mode-locking.
The vertical cavity surface emitting laser (VCSEL) is a well-known type of semiconductor laser. Its advantages include compactness, single axial mode emission, high quality circular beam shape, ease of mass production, and simple testability. Most VCSELs have a short resonator cavity, which limits their longitudinal or axial lasing modes to one.
Mode-locking is a known method capable of delivering short and high power pulses of radiation. Lasers with sufficiently long resonator cavities to support a significant number of axial lasing modes take advantage of mode-locking to produce a superposition of the axial modes yielding ultrashort pulses with very high peak powers. For more information on the theory of mode-locking and fundamental mode-locking techniques the reader is referred to Orazio Svelto, Principles of Lasers, translated by David C. Hanna, 4th edition, Plenum, pp. 330-364. A number of mode-locking techniques rely on a shutter or saturable absorber to mode-lock a number of the axial modes supported by the resonator cavity. Specifically, passive mode-locking takes advantage of the high peak power of the pulses as criteria for the saturable absorber to force the laser to run in mode-locked condition.
Because mode-locking is capable of generating a train of ultrashort pulses with high peak powers and low jitter it has many applications in a variety of fields. Short optical pulses have a large spectral bandwidth and can be used to generate multiple wavelength channels for telecommunication systems such as wavelength-division-multiplexed (WDM) and dense WDM (DWDM) optical communications networks. A high pulse repetition rate can also be utilized as a source for optical time-division-multiplexed (TDM) signals or for timing control signals in sampling applications. High repetition rate and low jitter mode-locked pulses can also be used for clock distribution in electronic systems.
It has been recognized that combining the advantages of VCSEL lasers with mode-locking would be of great benefit. In fact, the prior art teaches various structures and methods for using VCSELs in an external cavity mode-locking arrangement. For example, Jiang W., et al., xe2x80x9cFemtosecond Periodic Gain Vertical-Cavity Lasersxe2x80x9d, IEEE Photonics Technology Letters, Vol. 5, No. 1, January 1993, pp. 23-25 discloses an external cavity actively mode-locked VCSEL. This device is optically pumped by a mode-locked Ti:Sapphire laser. Jiang W., et al., xe2x80x9cElectrically pumped mode-locked vertical-cavity semiconductor laserxe2x80x9d, Optics Letters, Vol. 18, No. 22, November 1993, pp. 1937-1939 also teach an externally mode-locked VCSEL which is electrically rather than optically pumped. Hoogland S., et al., xe2x80x9cPassively Mode-Locked Diode-Pumped Surface-Emitting Semiconductor Laserxe2x80x9d, IEEE Photonics Technology Letters, Vol. 12, No. 9, September 2000, pp. 1135-1137 also teach an optically pumped VCSEL which is mode-locked using a saturable absorber mirror forming a part of an external cavity. For further examples the reader is referred to Dowd P., et al., xe2x80x9cMode-Locking of an InGaAs VCSEL in an External Cavityxe2x80x9d, LEOS 1995, IEEE, 8Th Annual Meeting Conference Proceedings, Vol. 2, pp. 139-140; Haring, R., et al., xe2x80x9cPassively Mode-Locked Diode-Pumped Surface-Emitting Semiconductor Lasersxe2x80x9d, CLEO 2000, Technical Digest (IEEE Cat. No. 00CH37088), pp. 97-98.
The disadvantages of using VCSELs in external cavity arrangements include large size, alignment problems and poor scalability. In fact external mode-locking is incompatible with one of the major advantages of VCSELs, namely the ability to manufacture them in dense arrays or integrate them into optoelectronic chips. Therefore, VCSELs that are mode-locked with an external cavity cannot be used in most of the desired applications that stand to gain the most from short, stable and high power pulses.
In accordance with another approach, it has also been proposed to lengthen the VCSEL structure and filter transverse modes that tend to naturally arise in long resonator cavities. This approach is discussed, e.g., by Nikolajeff F., et al., xe2x80x9cSpatial-mode control of vertical-cavity lasers with micromirrors fabricated and replicated in semiconductor materialsxe2x80x9d, Applied Optics, Vol. 38, No. 14, May 1999, pp. 3030-3038. This reference teaches the fabrication of micromirrors on substrates to spatially filter transverse modes in the far field for external cavity lasers and suggests ways of implementing the idea on monolithic cavities.
U.S. Pat. No. 5,574,738 to Morgan teaches yet another approach to derive high frequency pulses from a VCSEL. Specifically, Morgan teaches a GHz-range frequency-modulated laser using a VCSEL with a saturable absorber contained within the VCSEL""s distributed Bragg reflector to self-pulsate the VCSEL in the GHz regime. The repetition frequency is modulated with current, saturable absorber biasing or cavity design. The principles of self-pulsation are similar to those of Q-switching or spiking in which a build-up of population inversion while saturable absorber losses are high causes a high power laser pulse to develop when the saturable absorber losses drop. Thus, the self-pulsation technique taught by Morgan is implemented with a single axial mode in a short VCSEL.
In contrast with the phenomenon of self-pulsation used by Morgan, mode-locking requires a large number of axial modes to be supported by the VCSEL. In mode-locking the function of the saturable absorber is to absorb slow and low power components of the superposition produced the randomly phased axial modes. Meanwhile, fast and high power components of the superposition will saturate the absorber and pass through it. Thus, during mode-locking the saturable absorber induces the laser to yield high power mode-locked pulses.
The operation of the absorber in mode-locking is also in stark contrast with its operation in Q-switching, where it is used to prevent lasing in all modes while a population inversion is being build up. A drop in the absorption of the absorber upon saturation causes the laser to produce a pulse also referred to as giant pulse. The giant pulse is not a result of any particular superposition of axial modes. A Q-switched laser with a saturable absorber has build up times, as well as rise and fall times that depend on the cavity design and never reach mode-locking. In Q-switching the laser is not continuously on; lasing action is being turned on and off. In a mode-locked laser, on the other hand, all the modes are lasing continuously. It should also be noted that the repetition rates in mode-locking are determined by the cavity length while in Q-switching is determined by how fast can inversion be reached.
In fact, none of the prior art teachings can be used to devise a monolithically integrated mode-locked VCSEL, i.e., a VCSEL that is integrated in one device and does not require the use of an external cavity for mode-locking operation. That is because simply increasing the cavity size of a conventional VCSEL introduces significant problems related to resonator stability and dispersion. An additional problem relates to the bulk associated with the addition of mode-locking components, and associated loss of compactness. Therefore, it would be a major advance in the art of semiconductor lasers to provide a new type of VCSEL that combines the compactness and ease of mass production of conventional VCSELs with the advantageous properties of mode-locked lasers.
In view of the above shortcomings of the prior art, it is a primary object of the invention to provide a monolithically integrated VCSEL that can be mode-locked to deliver high frequency and high power ultrafast pulses. In particular, it is the object of the invention to ensure resonator stability in VCSELs with extended cavities and to compensate for dispersion to thus enable further pulse compression.
It is another object of the invention to provide for monolithically integrated mode-locked VCSELs, which are compact and easy to mass-produce.
These and numerous other advantages of the present invention will become apparent upon reading the following description.
The objects and advantages of the invention are achieved by a monolithically integrated, mode-locked vertical cavity surface emitting laser (VCSEL) in accordance with the invention. The resonator of the VCSEL has an active medium for emitting a radiation, a spacer for extending the resonator to support a significant number of axial modes of the radiation and a saturable absorber for establishing a certain phase condition between these modes to mode-lock them. The VCSEL has an arrangement for stabilizing the resonator such that diffraction losses are minimized and furthermore, only one transverse mode of the radiation is supported within the resonator. Additionally, the VCSEL is provided with an arrangement for compensating dispersion of the radiation occurring in the resonator.
The resonator is defined between a first reflector and a second reflector. Conveniently, at least one of these reflectors is a distributed Bragg reflector (DBR). In one embodiment of the invention the arrangement for stabilizing the resonator includes a specific curvature of the at least one DBR. In addition, the aperture of the at least one DBR is defined to ensure single transverse mode operation. Furthermore, the arrangement for stabilizing the resonator can include a resonator length as a geometric limitation. In fact, it is preferred that the length of the resonator be equal to twice the radius of curvature of the DBR reflector such that the resonator is a half confocal resonator. In the same embodiment or in a different embodiment the arrangement for compensating dispersion can be a chirp introduced into the at least one DBR reflector.
The arrangement for stabilizing the resonator can also include a lensing element. The lensing element can be provided independent of whether the resonator does or does not have a curved reflector. Preferably, the lensing element is a layered microlens embedded inside the resonator between the reflectors.
The monolithically integrated VCSEL and all of its components including the arrangements for stabilizing the resonator and dispersion compensation can be built on a single substrate. Preferably, the substrate is used as the spacer in this case. Any suitable pumping device can be used to pump the active medium residing in the resonator. The pumping device can be an electrical pumping device delivering a suitable current to the active medium. Alternatively, the pumping device can be an optical pumping device injecting pump radiation into the resonator to pump the active medium. When using an optical pumping device it is advantageous to place the saturable absorber at an opposite end of the resonator opposite from where the active medium is located. Also, the pump radiation should preferably be delivered through the reflector at the end where the active medium is located. The active medium can be any suitable lasing medium including a medium with quantum wells.
The invention further provides for a method of mode-locking the monolithically integrated VCSEL. Specifically, the resonator is preferably extended to support a significant number of axial modes of the radiation emitted by the active medium. The more axial modes are available, the more modes become available for mode-locking and producing the mode-locked output pulse. In some situations 5 or even fewer axial modes may be sufficient to obtain satisfactory pulses of output radiation.
A detailed description of the invention and the preferred and alternative embodiments is presented below in reference to the attached drawing figures.